Such heating coil fittings are used for the purpose of tightly connecting at least two installation elements, such as two pipes made of a synthetic material. For this purpose, the fitting pipe of the heating coil fitting, also produced from a synthetic material, has an interior heating coil that can be connected by means of two outwardly directed connection lines to an electrical power source. In order to tightly connect two synthetic pipes together, the pipes are slid into the fitting pipe, which has an interior diameter that is slightly greater than the outer diameter of the two pipes to be connected. Electric current is then directed to the heating coil, at which time part of the supplied power is converted into heat due to the inherent electrical resistance of the heating coil. This heat slowly penetrates, starting from the heating coil, the synthetic materials of the pipes and of the fitting. When the softening point or the melting range is exceeded, these materials break down or become soft and flow into each other. During cooling, they then harden forming a gas- or liquid-tight connection between the two pipes and the fitting. For this process, the term "heating coil welding" or "welding" is also used.
When a heating coil fitting is used for the purpose of tightly connecting the installation elements or mountings, of which at least one is made of a synthetic material, various aspects must be considered which are partially in opposition to each other. At first, it must be noted that the softening or melting temperature is exceeded for a sufficiently great volume at the connection point of, for example, the two synthetic pipes and the fitting. In other words, a thorough heating must occur at the two pipes and the fitting. If the heating is not thorough enough, the result is an insufficient stability after hardening, and if too thorough, this results in deformation during the warming or heating phase. Furthermore, a certain temperature above the softening or melting temperature must not be exceeded anywhere in the material(s), otherwise chemical aging or embrittlement of the synthetic material(s) occurs. Finally, during the welding process, a joining pressure must be built up in the connection or weld zone in order to assure sufficient cross-linking or a sufficient penetration of the material of the synthetic pipes and the fitting.
With the heating coil fittings known in practice, the above-mentioned joining pressure is achieved exclusively through the thermal expansion of the synthetic material and by the parasitic shrinkage of the injection-molded fitting pipes. Here, it has been shown to be disadvantageous that the width of the gap between, for example, two synthetic pipes and the heating coil fitting to be connected is not clearly defined prior to the fusion process. There may be gaps of several millimeters, particularly in the case of large heating coil fittings. This may lead to an insufficient thermal contact between the two installation elements to be connected and the heating coil fitting, as well as to an insufficient joining pressure, which necessarily leads to loose connections.
In U.S. Pat. No. 4,972,184, a heating coil fitting is disclosed whose pipe wall is constructed from two pipe-like layers. Here, along the external periphery of the first pipe-like layer, the second pipe-like layer is arranged coaxially. Between the two pipe-like layers, a helically extending heating wire is provided. The first layer, which is on the inside, is produced from a thermoplastic material. In contrast thereto, the external second layer of the fitting is produced from a thermosetting plastic material.
The installation elements to be connected with this known heating coil fitting are also constructed in two layers, wherein with respect to the material, the internal first layer of the installation elements corresponds to the external, second layer of the heating coil fitting and the external second layer of the installation elements corresponds to the internal first layer of the heating coil fitting. The external second layer of the installation elements, as well as the internal first layer of the heating coil fitting, assure, due to their thermoplastic material, a good material bond between the individual structyural components during the fusion process. In contrast thereto, the internal first layer of the installation elements, as well as the externally located second layer of the heting coil fitting, are to assure a sufficient stability of form. for this reason, particularly the latter layers must not undergo anyt change in form during the connection process, since otherwise a tight connection can no longer be assured. For this reason, in the case of relatively large gaps between the components to be connected, there exists the same problems with respect to an insufficient contact between the parts and the insufficient joining pressure, as already mentioned above in connection with the heating coil fittings known from practice.
Furthermore, the European Patent Application No. 0,378,406 also relates to a heating coil fitting with a two-layer construction, wherein the interior layer consists of a non-cross-linked polyolefin and the exterior layer consists of cross-linked polyolefin. The purpose of the interior layer, which is not cross-linked, is to assure a better bond between the material of this layer and the externally located material of the installation elements. Measures for eliminating dimensional tolerances are not discussed in this European Patent Application No. 0,378,406.
It is the task of the present invention to create a heating coil fitting as well as a manufacturing process for this purpose which makes possible a tight bond, independently of the diameter of the pipes to be connected and the fitting.